EXHIBIT 2

JOINT FILING AGREEMENT

The undersigned hereby agree to the joint filing of the Amendment No. 5 to Schedule 13D to which this Agreement is attached.

Dated:  September 1, 2011

DISCOVERY GROUP I, LLC

By  Michael R. Murphy*                                                 

       Michael R. Murphy

       Managing Member

Daniel J. Donoghue*

Daniel J. Donoghue

Michael R. Murphy*

Michael R. Murphy

*By: /s/ Mark Buckley                                                         

         Mark Buckley

         Attorney-in-Fact for Daniel J. Donoghue

         Attorney-in-Fact for Michael R. Murphy